Tracking (not Kerning)
Tracking is additional space between letters and glyphs, applied uniformly across a whole section of text.
How to Adjust Tracking
In your project's Fonts section, pick the style you want to edit, then adjust tracking using the -/+ buttons. You can also click on the tracking value and manually enter a number.
Negative tracking values work, too.
What does the ‰ symbol mean?
Per cent, or %, refers to one part in every hundred.
Per mille, or ‰, refers to one part in every thousand. Tracking is indicated in 1/1000ths of an em.
What is an em?
In typography, an em is a relative unit equivalent to your font size. Specifically, it is the width of a single em dash ("—") character.
A tracking value of 1000 would therefore be considered very large: it's the equivalent of inserting one em dash-sized space between each letter. Most of the time, tracking values are in the single or double digits.
Tracking is not Kerning
As Google points out, "Many novice typographers mistakenly talk about kerning when they actually mean tracking."
Tracking refers to space applied uniformly between all glyphs and characters. It's set by the font user (you).
Kerning is defined by the font designer, and defines custom adjustments to specific glyph pairs (like AV, AO, and LT).
Good fonts are always kerned, and kerning should always be turned on. Tracking is a separate creative choice applied by the designer.